The development of inhibitory antibodies to exogenous FVIII is an important complication of FVIII infusion in hemophilia A patients. Generation of such inhibitors might have the potential to preclude gene therapy for hemophilia A. In this project, we propose to investigate a novel approach for gene therapy of hemophilia A and hemophilia A with inhibitors based on the hypothesis that targeting the production of FVIII to a regulated secretory cell type (platelet) that activates at the site where FVIII is needed could overcome the presence of inhibitory antibodies. We have recently developed a vector (2bF8) that targets human FVIII expression to platelets under control of the platelet-specific 1IIb gene promoter. Using this construct, we can correct the murine hemophilia A phenotype even in the presence of high titer inhibitory antibodies in a transgenic mouse model. Another study using 2bF8 lentivirus-mediated bone marrow (BM) transduction and syngeneic transplantation resulted in therapeutic levels of FVIII expression in platelets in hemophilic mice with no antibody development. Our most recent studies have demonstrated that syngeneic transplantation of hematopoietic stem cells (HSC) from 2bF8 transgenic mice can efficiently restore hemostasis to hemophilic mice with inhibitors. In this R01 grant application, we propose to further explore whether 2bF8 lentiviral-mediated gene transfer can introduce sufficient platelet-FVIII expression and rescue the hemorrhagic phenotype in hemophilic mice and hemophilic sheep with inhibitors. To accomplish these, we first propose to transduce HSC from immunized FVIII null mice with 2bF8 lentivirus and transplant into immunized littermates. After BM reconstitution, the recipients will be analyzed by PCR, real-time PCR, LAM-PCR, confocal microscopy, FVIII activity assays, and inhibitor assays. Phenotypic correction will be assessed by tail clip survival test and in vivo thrombus formation. Secondly, we will transduce HSC from hemophilic sheep with or without inhibitors using 2bF8 lentivirus and then transplant the 2bF8-transduced autologous cells back into the animals. The treated animals will be analyzed by the assays as described above, plus whole blood clotting time, thrombin generation assay and thromboelastography assay. Treated animals will be closely monitored to determine if the hemorrhagic phenotype has been improved. Lastly, we will transplant the 2bF8-transduced sheep HSC into hemophilic sheep fetuses to investigate the efficacy and safety of fetal gene therapy using 2bF8 lentiviral vector. These studies should allow us to address the efficacy and safety of our lentivirus-mediated platelet-specific gene therapy approach in animal models with anti-FVIII specific immunity, with the potential to develop a long- term strategy for gene therapy of hemophilia A patients and patients with inhibitors.